1. Field of the Invention
The present invention relates to a catalyst comprising a support in the form of beads based on alumina, at least one catalytic metal or a compound of a catalytic metal from group VIB (column 6 in the new periodic table notation), optionally at least one catalytic metal or a compound of a catalytic metal from group VIII (columns 8, 9 and 10 of the new periodic table notation), with a pore structure composed of a plurality of juxtaposed agglomerates, each formed by a plurality of acicular platelets, the platelets of each agglomerate being generally radially orientated with respect to the other and with respect to the center of the agglomerate, said catalyst being characterized in that it further contains at least one doping element selected from the group constituted by phosphorus, boron, silicon and the halogens.
The present invention also relates to a process for preparing said catalyst.
The present invention also relates to the use of said catalyst in processes for converting metal-containing hydrocarbon feeds.
2. Description of Related Art
The skilled person is aware that during reactions for hydrorefining and/or hydroconverting petroleum fractions containing organometallic complexes, the majority of those complexes are destroyed in the presence of hydrogen, hydrogen sulfide, and a hydrotreatment catalyst. The constituent metal of such complexes then precipitates out in the form of a solid sulfide which then becomes fixed on the internal surface of the pores. This is particularly the case for vanadium, nickel, iron, sodium, titanium, silicon, and copper complexes which are naturally present to a greater or lesser extent in crude oils depending on the origin of the crude and which, during distillation, tend to concentrate in the high boiling point fractions and in particular in residues. This is also the case for liquefied coal products which also comprise metals, in particular iron and titanium. The general term hydrodemetallization is used to designate those organometallic complex destruction reactions in hydrocarbons.
The accumulation of solid deposits in catalyst pores can continue until a portion of the pores controlling access of reactants to a fraction of the interconnected pore network is completely blocked so that said fraction becomes inactive even if the pores of that fraction are only slightly blocked or even intact. That phenomenon can thus cause premature and very severe catalyst deactivation. It is particularly sensitive in hydrodemetallization reactions carried out in the present of a supported heterogeneous catalyst. The term “heterogeneous” means not soluble in the hydrocarbon feed. In that case, it has been shown that the pores on the periphery are blocked more quickly than the central pores. Similarly, the pore mouths block up more quickly than their other portions. Pore blocking is accompanied by a gradual reduction in their diameter, which increasingly limits molecule diffusion and increases the concentration gradient, thus accentuating the heterogeneity of the deposit from the periphery to the interior of the porous particles to the point that pores opening to the outside are very rapidly blocked: access to the practically intact internal pores of the particles is thus denied to the reactants and the catalyst is prematurely deactivated.
The phenomenon described above is known as pore mouth plugging. Proofs for its existence and an analysis of its causes have frequently been published in the international scientific literature, e.g.:                “Catalyst deactivation through pore mouth plugging” presented at the 5th International Chemical Engineering Symposium at Houston, Tex., U.S.A., March 1978, or “Effects of feed metals on catalyst aging in hydroprocessing residuum” in Industrial Engineering Chemistry Process Design and Development, volume 20, pages 262 to 273 published in 1981 by the American Chemical Society, or more recently in “Effect of catalyst pore structure on hydrotreating of heavy oil” presented at the National conference of the American Chemical Society at Las Vegas, U.S.A., 30th Mar. 1982.        
A catalyst for hydrotreating heavy metal-containing hydrocarbon cuts must thus be composed of a catalytic support with a porosity profile which is particularly suitable for the specific diffusional constraints of hydrotreatment and with a texture adapted to proper capture of metallic impurities, while avoiding the plugging problems mentioned above.
The catalysts usually used are composed of a alumina-based support having a particular porosity and an active phase based on mixed sulfides constituted both by a sulfide of a metal from group VIB (preferably molybdenum) and a sulfide of a metal from group VIII (preferably Ni or Co). The metals are deposited in the oxide state and are sulfurized so that they are active for hydrotreatment. The atomic ratio between the element from group VIII and the element from group VIB which is usually considered to be optimal is a group VIII/group VIB ratio in the range 0.4 to 0.6. Recently, it has been shown in European patent EP-A1-1 364 707 (FR-A-2 839 902) that independently of the porous texture, a ratio of less than 0.4 can limit deactivation of the catalysts and thus extend the service life of the catalysts.
The skilled person is aware that two types of alumina based support exist for catalysts for hydrorefining and/or hydroconverting heavy metal-containing hydrocarbon feeds. Those supports are distinguished by their pore distribution profile.
The first, which is bimodal in nature, is characterized by the presence of two distinct families of pores, one in the mesopore range, the other in the macropore range. Mesopores are defined as pores with a diameter of less than 500 Å and macropores are pores with a diameter of more than 500 Å, the porosity being measured by the mercury intrusion method. The function of mesopores is to develop a large specific surface area, which multiplies the chances of contact between reactive molecules and catalytic sites. The macropores act to irrigate the set of mesoporous domains (where the active phase is concentrated) by reactive molecules and to carry out the asphaltene disaggregation reaction to reduce their size.
That type of bimodal support can be prepared from a gel of boehmite or pseudoboehmite type alumina or by co-mixing a γ alumina feed or alumina calcined with a binder which may be a boehmite or pseudoboehmite type alumina (U.S. Pat. No. 5,089,463—Chevron) or an organic compound such as cellulose acetate (EP-A1-1 060 794, Japan Energy Corporation) or by co-mixing a boehmite or pseudoboehmite type alumina gel with recycled fines (U.S. Pat. No. 5,827,421—Texaco). The active phase can be introduced in the oxide state either by impregnating a solution of salts of the elements to be deposited (U.S. Pat. No. 5,827,42113 Texaco), or during preparation of the support (U.S. Pat. No. 4,880,525—Shell). These catalysts are usually in the form of extrudates, but can also be in the form of beads (U.S. Pat. No. B1 6,656,349—Nippon Ketjen).
The impact of the pore distribution of such bimodal catalysts has been widely studied. According to patent EP-A-1 060 794, the presence of at least 0.32 cm3/g of macropores (over 500 Å) and of medium size mesopores in the range 80 to 200 Å can produce a catalyst having both a high initial activity and a high metal retention capacity as well as a long service life. The proportion of macropores has been the subject of debate since, according to U.S. Pat. No. 5,397,456, there exists a compromise between increasing the macroporosity to encourage diffusion of large molecules and reducing the macroporosity to limit poisoning inside the grains. According to those authors, 11% to 18% of the pore volume present in pores larger than 250 Å is a good proportion. Further, according to the U.S. Pat. No. 5,827,421, a catalyst having a large pore volume (0.82 to 0.98 cm3/g) and the feature of having large mesopores (55 to 64.5% of the pore volume between 110-130 Å±50 Å) and a large fraction of macropores (27% to 34% of the pore volume above 250 Å) is particularly good when maximizing the converting said feeds while limiting the formation of sediments in the conversion products and maximizing desulfurization.
Such catalysts with a bimodal porosity profile are highly active, but have a poorer retention capacity than catalysts with a polymodal porosity profile.
The second porosity profile for alumina-based supports for hydrorefining and/or hydroconverting metal-containing hydrocarbon feeds is the polymodal nature. The cumulative distribution curve as a function of the pore diameter obtained by the mercury intrusion method is neither monomodal nor bimodal in that no distinct pore families appear with pore diameters centered on well defined mean values, but rather, a relatively continuous distribution of pores between two extreme diameter values. Between those two extremes, there is no horizontal plateau on the pore distribution curve. Said polymodal distribution is linked to a “chestnut burr” or “sea urchin” pore structure obtained with alumina agglomerates prepared by rapid dehydration of hydrargillite then agglomeration of the flash alumina powder obtained, according to one of the Applicant's patents (U.S. Pat. No. 4,552,650—IFP). The prepared alumina agglomerates can be in the form of beads or in the form of extrudates, as shown in patents FR-A-2 764 213 and U.S. Pat. No. 6,043,187.
The chestnut burr or sea urchin structure is constituted by a plurality of juxtaposed agglomerate each formed by a plurality of acicular platelets, the platelets of each agglomerate being orientated generally radially with respect to each other and with respect to the center of the agglomerate at least 50% of the acicular platelets have a dimension along their longest axis in the range 0.05 to 5 micrometers, preferably in the range 0.1 to 2 micrometers, a ratio of said dimension to their mean width in the range 2 to 20, preferably 5 to 15, a ratio of said dimension to their mean thickness in the range 1 to 5000, preferably in the range 10 to 200. At least 50% of the acicular platelet agglomerates constitute a collection of pseudo-spherical particles with a mean size in the range 1 to 20 micrometers, preferably in the range 2 to 10 μm. Very satisfactory images for representing such a structure is a pile of resinous chestnut burrs, or a pile of sea urchins, hence the name of the pore structure—chestnut burr or sea urchin—applied by the skilled person.
The majority of pores is constituted by free spaces located between radiating acicular platelets. Because of their “wedge” nature, those pores have a continuously variable diameter of between 100 and 1000 Å. The network of interconnecting macropores results from the space left free between the juxtaposed agglomerates.
Said catalysts with a polymodal porosity profile have a pore distribution (determined by the mercury porosimetry method) which is preferably characterized as follows:                total pore volume in the range 0.7 to 2 cm3/g;        % of total pore volume as pores with a mean diameter of less than 100 Å: between 0 and 10;        % of total pore volume as pores with a mean diameter in the range 100 to 1000 Å: between 40 and 90;        % of total pore volume as pores with a mean diameter in the range 1000 to 5000 Å: between 5 and 60;        % of total pore volume as pores with a mean diameter in the range 5000 to 10000 Å: between 5 and 50;        % of total pore volume as pores with a mean diameter of more than 10000 Å: between 5 and 20.        
The specific surface area for said catalysts, measured by the BET method, is in the range 50 to 250 m2/g.
The chestnut burr or sea urchin pore structure associated with the pore distribution characteristics described above can produce hydrorefining and/or hydroconversion catalysts with very high retention powers, while keeping a high hydrodemetallization activity, which are not possible with bimodal catalysts. The reasons are that the “wedge” shape of the mesopores of the chestnut burr or sea urchin structure compensate for or remove the reactant concentration gradients which would normally be established in a cylindrical pore, which is supplemented by a geometry which is highly favorable for discouraging pore mouth clogging. Further, every mesopore or nearly every mesopore has independent access to the interstitial macropores, encouraging homogeneous accumulation of deposits without premature deactivating clogging.
Said catalysts, however, suffer from the disadvantage of being less active as regards initial activity than bimodal catalysts for HDM (hydrodemetallization), HDAC7 (hydroconverting asphaltenes that are insoluble in n-heptane), and HDCCR (hydroconverting carbon-containing residues quantified by Conradson carbon analysis). This initial activity criterion is currently important both for fixed bed residue hydrorefining and for ebullated bed residue hydroconversion.
In fixed bed residue hydrorefining processes, although said HDM catalysts have a high retention power, necessary for processing hydrocarbon feeds with high metals contents (Ni+V over 40 ppm, e.g.), the poorer initial performances for the HDAC7, HDM, HDCCR functions of this type of catalyst are deleterious to the performances of the downstream HDS catalysts, which are thus poorly protected from asphaltenes, deposition of the metals Ni+V and coke.
In ebullated bed hydroconversion processes processing hydrocarbon feeds with high metal contents (Ni+V of more than 250 ppm, for example), the initial poorer performance of said catalyst with a chestnut burr pore structure or sea urchin pore structure renders necessary an increasing daily quantity of fresh makeup catalyst.
For hydrocarbon feeds containing high metals contents, the bead form is preferred to the extrudate form both for fixed bed hydrorefining and for ebullated bed hydroconversion.
For fixed bed hydrorefining, the bead form of the HDM catalyst generates less initial delta P (loss on ignition), allowing the catalyst cycle period to be extended. The bead form also allows the catalyst to be discharged more easily because intergranular plugging is reduced. Further, size grading of the beads is easy to carry out and allows the delta P of the fixed bed to be reduced while maintaining good metal retention (Ni+V). The catalyst in the bead form is preferred in the context of an HDM application.
For ebullated bed hydroconversion, the bead form allows more homogeneous bed fluidization and has improved abrasion resistance properties compared with the extruded form. Bead movement is more homogeneous and the homogeneity of the solids in the bed produces good metal retention by avoiding segregation due to gravity. The bead size can also be adjusted as a function of the desired chemical activity to minimize problems linked to diffusion of molecules in the pores of the catalyst. Metals capture is considerably increased in an ebullated bed compared with a fixed bed.
Surprisingly, the Applicant has discovered that adding a doping element selected from the group formed by phosphorus, boron, silicon and the halogens to polymodal catalysts with a chestnut burr type texture in bead form can increase the initial activity of said catalysts, and thus provides better protection for the catalysts located downstream in fixed bed processes, but can also reduce the daily quantity of fresh makeup catalyst used in ebullated bed processes.